The invention relates to a method for conveying pulverulent material which is drawn in by a conveying gas flow to an intake chamber and at least one adjustable second gas flow is introduced in to the conveying gas flow. The invention also relates to an application of the injector arrangement in which the combined gas and powder material mixture are directed onto the weld seam of a can body to form a coating on the seam.
It is known to convey pulverulent materials by means of an injector, or an injector arrangement comprising an injector and a compressed air source. In particular, for conveying of pulverulent coating powder, it is known to use an injector in which, after the introduction of the coating powder, dosing air is fed into the conveying air.
FIG. 1 shows such an injector 1 according to the state of the art. In this, air is blown through a nozzle 4 into the injector chamber 5, which has a powder intake 6 for introducing the powder into the conveying jet from a feed container which is not shown. The powder transported in the air jet passes into the chamber 10, where dosing air is fed to the powder stream via a connection 8 and a duct 9. The powder/air mixture is directed to the coating point through a hose connected to the hose connection 11, and conduits. The conveying air and dosing air, which are drawn from a conventional compressed air source, have hitherto been controlled either by separate control valves or by two valves mounted on a common shaft so that both valves could be adjusted by operating a single control knob.
Especially in applications where the powder/air mixture has to travel along a relatively long conduit to the coating point (eg. a distance of 1 metre or more) or where a highly homogeneous powder/air mixture is required, controlling the conveying air and dosing air has hitherto proved very difficult. Where material is carried over long distances and/or a high degree of homogeneity is required, the proportioning between conveying air (which determines the quantity of powder removed from the feed container) and dosing air (which affects the velocity of the powder/air mixture in the conduit and the homogeneity of the mixture) is very difficult to set correctly, and even using two coupled control valves it has been found that a satisfactory setting is possible only within a narrow operating range.
Conditions are particularly awkward in the known process for coating weld seams of can bodies at the end of a can body welding machine. For one thing, the supply of powder by means of the injector has to pass through an extended conduit, as the conduit has to be routed through the welding machine along the body forming and welding path. For another thing, in order to obtain a weld seam coating of good quality, the powder must be delivered onto the can body at a constant rate and with a uniform distribution as the body is conveyed past the coating nozzle. Can bodies are conveyed at a rate of eg. 18 cans per second (or, for a standard size can, at approximately 100 m/min), so that any fluctuation, even momentary, in the homogeneity of the powder/air mixture or in the absolute quantity of powder delivered may result in a large number of cans receiving an insufficient coating.
It has therefore already been proposed that the conveying air and dosing air should be drawn from a source with a constant conveying rate of flow. This solution, however, displays an unsatisfactory reaction if there is disturbance in the conduit. Pulsation may occur in the conduit and the resulting fluctuations may lead to accumulations of powder in the conduit.
Therefore the problem which lies at the basis of the invention is to provide a way of coating by means of an injector which avoids these drawbacks and in which, even under very difficult circumstances, and particularly in the coating of the weld seam of can bodies, excellent coating quality can be obtained.